Grayscale image data is obtained by scanning an image with a suitable device, for example, a scanner comprising a plurality of charge-coupled devices (CCD's). The scanner effectively divides the image into a finite number of small picture elements referred to as pels or pixels. Each pixel of a scanned image is converted into a number representative of the grayscale value of the pixel as detected by a CCD scanner cell.
The image is ultimately to be applied to a bilevel device i.e., a device which reproduces each pixel as one of two grayscale values for duplication or display of the image. For a typical bilevel image duplication device, the two grayscale values are black or white and correspond to print or no print picture elements, respectively.
The determination of which pixels are to be printed or left not printed for duplication of a given image depends to some extent on the characteristics of the image. For example, portions of the image may be classified as either continuous tone or text. Photographs are an example of a continuous tone image while text is exemplified by line drawings and letter images.
For text data, the conversion from grayscale values to binary is often accomplished by establishing a threshold to which the grayscale value of each pel is compared. The result of the comparison is that if the grayscale value exceeds the threshold, a block or print representation is selected for the pel and conversely, if the threshold is not exceeded, the pel is left white or not printed.
For continuous tone image data, alternate techniques are applied. For example, a variety of pel block patterns may be selected to represent blocks of the continuous tone image data dependent upon the composite grayscale value of the blocks. For continuous tone image data, pel block patterns may be shifted, rotated or otherwise varied to prevent the appearance of interference patterns such as Moire patterns.
Use of a digital filter is also common in imaging systems to "enhance" an image before thresholding. Typically a one dimensional or two dimensional Finite Impulse Response (FIR) convolution filter is used. The FIR is programmed such that the sum of the coefficients is equal to one, thereby yielding unit gain at DC (zero frequency) so as to preserve the tonal qualities of the image.
U.S. Pat. No. 4,723,173 entitled IMAGE PROCESSING APPARATUS and U.S. Pat. No. 4,288,821, entitled MULTI-RESOLUTION IMAGE SIGNAL PROCESSING APPARATUS AND METHOD. In both patents, a decision is made of whether to use "line art" having a first threshold or whether a "continuous tone" algorithm should be used to process the image data. These types of discriminators that switch between two different processing circuits depending on the type of data occurring at the input can be highly complex and often requiring more circuitry than the two thresholding circuits combined. Image processors of this type tend to utilize separate circuitry for processing "line art" (text material) and continuous tone images. These processes tend to respond differently for black to white and white to black transitions.